"Surely Marcus does not live in New York?"
Translation:Num Marcus Novi Eboraci habitat?
Marce is the vocative for Marcus. You use vocative when directly addressing a person.
In the sentence above, you are talking about Marcus, but not to him.
"The vocative case presents little problem for English speakers. It is usually the same as the nominative, as in English, and it is used when you address someone directly.
"The exceptions to the rule that the vocative is the same as the nominative are summarized in the phrase, Marce mi fili, which is the vocative for Marcus meus filius, and is a convenient way to remember that all 2nd declension nouns in -us, have a vocative in -e, that the vocative of meus is mi, and that all 2nd declension nouns in -ius have a vocative in -i."